1. Field of the Invention
The present invention relates to a device and a method for pretreating a signal to be transferred using a non-linear amplifier which is subjected to a prelinearization and a predistortion, respectively, in order to counteract non-linearities introduced by the non-linear amplifier. In particular, the present invention relates to a device and a method for pretreating a signal which is to be transferred over a non-linear amplifier which has a preceding narrow-banded band-pass filter whose pass-band width is only slightly larger than the signal bandwidth to be processed.
2. Description of Prior Art
It is known to subject signals to a predistortion and prelinearization, respectively, which are to be transferred using a non-linear amplifier, for example using a non-linear amplifier which is arranged in a communication satellite, in order to counteract the non-linearities generated by the amplifier and completely compensate them in the optimum case, respectively. In the case of a satellite transmission system it is attempted to compensate distortions this way which occur in the output amplifier of the satellite, by a corresponding predistortion of the transmission signal in the ground station.
An exemplary method for generating a linearized amplifier output signal in which distortions generated by a non-linear amplifier are reduced and compensated, respectively, responding to a predistorted input signal is described in U.S. Pat. No. 5,049,832. In this method the amount square of the input signal is detected, wherein a table is accessed depending on the amount square in order to multiply the input signal with a predetermined complex coefficient in order to cause a predistortion of the input signal, such that the output signal amplified by the non-linear amplifier is linearized according to amount and phase. A means for comparing input signal and output signal is provided in order to perform an adjustment of the coefficients stored in the table.
An alternative method for performing a predistortion for a non-linear transmission link in the high-frequency range is known from EP 0885482 B1, wherein an envelope curve of an input signal is detected and quantized in the method described there, in order to generate quantized envelope curve values. On the basis of detected, quantized envelope curve values a table means is accessed in which complex predistortion coefficients are stored. The input signal is predistorted on the basis of the predistortion coefficients, such that the distortion introduced by the non-linear transmission link is basically compensated according to amount and phase.
A still further method for performing a predistortion and prelinearization, respectively, of a transmission signal to be transferred over a non-linear transmission link is disclosed in the not pre-published German Patent Application 19927952.7. In the method illustrated there an estimation signal is generated using a table in which complex coefficients are stored, at first depending on an input signal, wherein the estimation signal illustrates an estimation of an output signal amplified by a non-linear amplifier. A difference is formed from the estimation signal and the input signal illustrating an error signal illustrating the estimation of the error introduced by the non-linear amplifier. This error signal is time-expanded by a time-dispersive element and then subtracted from the input signal in order to perform a pre-linearization of the input signal, such that an error signal part in the frequency spectrum of the output signal is shifted away from the useable frequency range of the signal.
The above-mentioned predistortion and prelinearization methods, respectively, and all further predistortion methods of a plurality known in the art, wherein a predistortion and prelinearization, respectively, of a signal is executed before the transmission of the same by a non-linear amplifier, will fail when the bandwidth of a preceding channel filter to the non-linear amplifier lies essentially in the range of the signal bandwidth to be processed or amounts to less than three to five times the bandwidth of the undistorted signal, respectively.